There are various well-known catalytic formulations allowing to convert nitrogen oxides in the exhaust line before the exhaust gases are discharged to the atmosphere.
The most commonly used formulations are Platinum/Alumina, which are especially active at low temperatures (of the order of 200.degree. C.) and the copper/zeolite formulations, which are active at higher temperatures (of the order of 350.degree. C.).
However, the efficiency of nitrogen oxides reduction in an oxidizing medium depends on the running conditions of the engine and notably on the temperature and the flow of the exhaust gases.
The ratio of the reducers contained in the exhaust gases to the nitrogen oxides (NO.sub.x) is also very important, as explained hereafter.
The reducers involved in the catalytic reduction of nitrogen oxides in an oxidizing medium are mainly hydrocarbons (HC).
A problem arises when these reducers are not in sufficient quantities to ensure proper conversion of the nitrogen oxides. Diesel engines for example present this type of drawback. The HC/NO.sub.x ratios are then far less than one, whereas the activity of the catalysts is really significant only with HC/NO.sub.x ratios greater than one.
There are various known means allowing to increase the hydrocarbon content of exhaust gases.
It is well-known to inject hydrocarbons directly into the exhaust line, upstream from the catalytic conversion. A specific system, independent of the engine, is then used.
Engines equipped with a high-pressure fuel injection system referred to as &lt;&lt;common rail&gt;&gt; allow controlled hydrocarbon injections in the engine itself. Pilot injections of fuel before or after the main injection allow to change the composition of the gases at the exhaust and notably the reactivity to the catalyst.
The hydrocarbons can thus be significantly increased at the exhaust and a sufficient concentration can be obtained to reduce the nitrogen oxides on the catalyst.
It is well-known that the catalytic formulations used for nitrogen oxides reduction in an oxidizing medium are characterized by a relatively narrow temperature range, so that the major part of the operating process does not get a satisfactory conversion.
Known injections of additional fuel at the exhaust are furthermore performed continuously, which implies a certain &lt;&lt;additional consumption&gt;&gt;.
In order to remedy these drawbacks and to improve the activity of a deNO.sub.x catalyst, it is possible, according to the invention, to adopt a particular hydrocarbon injection strategy.